1. Field of the Invention
The invention pertains generally to fiber optic couplers and in particular to the correction of the birefringence of such couplers.
2. Description of the Prior Art
The description of the prior art is divided into four parts: (1) definitions of fiber optic couplers and two special types of couplers, namely combiners and splitters; (2) a description of the birefringence or change in polarization introduced by most single mode couplers; (3) a presentation of the Jones calculus for two-part birefringent systems; and (4) a discussion of the usefulness of the prior art for eliminating birefringence in single mode optical couplers.
Single mode optical fibers are finding increasing application in interferometric sensors such as acoustic, magnetic, and rotational sensors in data bus distribution systems for communications applications. Such uses often require single mode fiber couplers which couple together the signals on two or more fibers. A simple type of single mode directional coupler described by Bergh et al in Electronics Letters, volume 16, pages 260-261, 1980 involves forcing together two parallel fibers. Light modes propagating on either of the fibers will couple in a controlled fashion onto the other fiber and propagate in the parallel direction on that fiber. Such a coupler is most generally a 2.times.2 coupler since it has two input ports and two output ports. A 2.times.2 coupler can also be used as a 1.times.2 coupler or splitter which has a single input port but two output ports through which the signal propagates. Alternatively the 2.times.2 coupler can be used as a 1.times.2 combiner which has two input ports and a single output port through which propagates a signal which is a combination of the input signals. More general combiners can be built with more input or output ports. In particular, it is possible to build 1.times.N splitters and N.times.1 combiners. In fact, couplers are usually reciprocal devices, i.e. they can operate backwards in the respect that if a given combination of signals on the input ports produces a given output signal at the output port, then if that given output signal is actively applied to the output port (now used as an input), the original combination of input signals appears at the input ports (now used as outputs). As a result a reciprocal combiner can be used as a splitter and vice versa.
Single mode fiber couplers of the type described by Bergh are being fabricated by a variety of methods, such as etching, polishing, and fusion and can be encapsulated in liquid, epoxy, sol-gel glass or left unencapsulated. Often the fiber leads are fixed with epoxy to facilitate mounting in a container after the coupler is fabricated. Although little data has been published, it has become evident that these types of couplers typically do not preserve optical polarization through the coupler. Instead linearly polarized inputs are often transformed to highly elliptical states of polarization upon exiting the coupler. Analagous transformations occur for circularly polarized input. Since these couplers are often used in series and the coupling fraction typically varies with the input polarization state, these effects lead to reduced sensitivity and polarization noise in interferometers and to non-optimum coupling fractions in fiber data buses. The mechanism for polarization transformation within a fiber coupler appears to be due to the presence of linear and circular birefringence induced in the fibers during the process of coupler fabrication, and also to the intrinsic birefringence of the fiber introduced during the fiber's manufacture. The process of how these birefringences alter the state of optical polarization of the light passing through a fiber has been explained in detail by Ulrich and Simon in Applied Optics, volume 18, pages 2241-2251, 1979. This understanding of the mechanisms will probably enable the future fabrication of single mode couplers with less birefringence. To what degree this can be accomplished in the future is uncertain and it is unknown if such fabrication techniques will impose additional constraints upon coupler design. It is possible that a coupler can be made that is free of birefringence for one set of conditions but becomes birefringent with further aging or a change of environmental conditions.
In a different approach, Hurwitz and Jones have shown in the Journal of the Optical Society of America, volume 31, pages 493-499, 1941 that for monochromatic input a two port system of arbitrary birefringence can be represented by two lumped biefringent elements: a linear birefringence elements (retarder) with retardation R and fast axis orientation .phi., and a circular birefringent element (rotator) with rotation .OMEGA..
FIG. 1a is a pictorial representation of a two-port lossless system 10 which may be simply an optical fiber. The system 10 has an input port 12 and an output port 14. Monochromatic signal inputs to a system 10 which is a single PG,6 mode system can be represented by the input vector E.sub.in while the output vector is E.sub.out. The basis of these two vectors are orthogonal light modes, here the two linear polarizations. Hurwitz and Jones showed that the system 10 can be represented by the lumped birefringent elements shown in the equivalent representation of FIG. 1b. The general birefringent system 10 can be replaced by a retarder 16 having retardation R and a fast-axis orientation .phi. for that retardation followed by a rotator 18 having rotation .OMEGA.. Note that the order of the elements 16 and 18 is important. The elements 16 and 18 may be reversed but the values of their parameters will in general be different. According to the Jones calculus described by Jones and Hurwitz the two elements 16 and 18 may be represented by the two matrices ##EQU1## The transformation represented in Eqn. 1 is a rotation of the axes by -.OMEGA. which is equivalent to a rotation of the optical field by +.OMEGA.. Jones and Hurwitz then showed that the two-part system of FIG. 1b is equivalent to the mathematical transformation of ##EQU2## This equation is a vector transformation involving multiple matrix multiplications.
The usefulness of the lumped birefringent elements 16 and 18 for multi-port (greater than two) systems was not disclosed by Jones and Hurwitz. Their derivation depended upon a single optical path, a model incompatible with dividers and combiners. Indeed it has been observed theoretically that when the coupling between optical fibers is anisotropic, i.e. depends on the state of polarization, then the model of lumped birefringent elements cannot represent such a path.
Ulrich has described in Applied Physics Letters, volume 35, pages 840-842, a polarization stabilization scheme for a single optical fiber. The fiber was understood to be undergoing time-varying environmental changes of its birefringence. Therefore a complicated feed-back system was used to correct the fibers birefringence in real-time. The system furthermore could only work with a single polarization mode on the single mode fiber.